What phenomenon occurs in a buffer when small amounts of acid or base are added?

When small amounts of acid or base are added to a buffer solution, the pH remains relatively stable due to the nature of how buffers work. A buffer consists of a weak acid and its conjugate base (or a weak base and its conjugate acid), which are present in equilibrium.

When an acid is added, the weak base component of the buffer reacts with the added hydrogen ions (H⁺) to form more of the weak acid, minimizing changes in pH. Conversely, if a base is added, the weak acid component can release hydrogen ions to counteract the increase in hydroxide ions (OH⁻), thus stabilizing the pH.

This ability to resist significant changes in pH is a defining characteristic of buffer solutions, allowing them to maintain a relatively constant pH even in the presence of external acids or bases, as long as the amount added does not exceed the buffer's capacity.